1. Field of the Invention
The present invention relates to a bearing and a screw compressor. Particularly, the present invention is concerned with a bearing superior in corrosion resistance and a screw compressor having the bearing superior in corrosion resistance.
2. Description of the Related Art
As known well, a screw compressor has a casing for housing therein a pair of female and male screw rotors meshed with each other. End portions of rotor shafts of each of the pair of screw rotors are supported by radial bearings. On the other hand, a pair of thrust bearings for bearing a thrust force developed on the screw rotor are mounted at one end of a rotor shaft, in each of the pair of female and male screw rotors. The thrust bearings are disposed on both sides of a disc-like thrust member fitted on the one end of the rotor shaft in each of the pair of screw rotors. The thrust bearings are in sliding contact with a slide surface of the thrust member to bear a thrust force transmitted from the screw rotor to the thrust member. In the screw compressor of the above construction, since the screw rotors are rotated at high speed, plain bearings are used as the radial bearings and the thrust bearings. Since slide portions of the bearings are required to be low in friction, a low friction material is used for particularly slide portions of the thrust bearings which receive a thrust force from the thrust members of a high peripheral velocity.
As a thrust bearing using a low friction material for its slide portion, one constituted for example by a plural-layer slide member is known. In this plural-layer slide member, a porous intermediate layer is formed between a metallic backing strip difficult to bond and a resin layer as a slide member to let an anchoring effect be exhibited, thereby enhancing the bonding force between the metal and the resin layer to make the resin layer difficult to be peeled and improve the abrasion resistance and sliding characteristic of the resin layer. The bearing constituted by the plural-layer slide member is disclosed, for example, in Japanese Patent Laid Open publication No. 2002-194380.
The above plural-layer slide member is difficult to peel from the backing strip and is superior in both abrasion resistance and sliding characteristic, so is considered suitable as a constituent material of a thrust bearing.
If a thrust bearing constituted by such a plural-layer slide member is used for example on a rotor shaft of a screw rotor in a screw compressor, gas contained in an atmosphere, i.e., suction gas, and lubricating oil get in around the thrust bearing, so that the thrust bearing comes into contact with the gas and the lubricating oil. In this case, if a corrosive component, e.g., ammonia, which corrodes components of the thrust bearing is contained in the gas or the lubricating oil, the porous intermediate layer, which contains copper, will be corroded, with consequent damage of the thrust bearing, thus requiring a frequent repair of the screw compressor. As a result, not only the rate of operation of the screw compressor becomes lower, but also the cost for maintenance of the screw compressor becomes higher.